This is far more than previous estimates of distinguishable olfactory stimuli, or 10,000 odors, – reports this Science paper. The approach is fascinating in itself, as the Science editor summarized:
“Because the authors reduced the complexity by investigating only mixtures of 10, 20, or 30 components drawn from a collection of 128 odorous molecules, this astonishingly large number is probably the lower limit of the potential number of olfactory stimuli that humans can distinguish.”
I have been always fascinated by smells as I like perfume and have a good olfactory memory. Smells, or odorants (odorous molecules), are detected by binding to specialized olfactory receptor neurons lining the nose in the olfactory epithelium, which is about 10 squared cm in human and contains millions of neurons. Each neuron expresses only one functional odor receptor, expressed by a single gene; humans have 350 such genes.
“Odor sensation depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant stimulus type is typically recognized by multiple receptors, and different odorants are recognized by combinations of receptors, the patterns of neuron signals helping to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix, not necessarily corresponding to concentration or intensity of any single constituent.” (Wikipedia) There are different theories how exactly the stimulation happens.
Out of the 117 known chemical elements, only 87 can form chemical compounds, estimated number of which is from 10^18 to 10^200 (for comparison the number of grains of sand on the Earth is about 7.5 x 10^18, the number of particles in the universe is between 10^72 and 10^87) (the numbers are from wisegeek). Aromatic compounds are stable and abundant in both natural and synthetic forms. But how many of them are known? Surprisingly difficult to come up with a complete picture searching Google.